1. Field of the Invention
The present invention relates to a method of inhibiting the formation of coke in a pyrolysis cracker, and more particularly, to a method of inhibiting the formation of coke in an ethylene dichloride to vinyl chloride monomer pyrolysis cracker.
2. Description of the Related Art
Pyrolysis crackers are typically operated at temperatures of from about 400° C. to about 600° C., at gauge pressures of from about 1.4 Mpa to about 3.0 Mpa and with a residence time from about 2 seconds to about 60 seconds. Ethylene dichloride (EDC) conversion per pass through a pyrolysis cracker is normally maintained around 50–70% with a selectivity of 96–99% to a vinyl chloride product. In this case, vinyl chloride monomer (VCM) and HCl are produced. By-products from the pyrolysis process range from the very lights, such as methane, acetylene, ethylene, and methyl chloride, to the heavies, such as carbon tetrachloride, trichloroethane and solid carbonaceous material. Solid carbonaceous material is usually referred to as coke, and coke brings about problems.
Higher conversion in the pyrolysis process is, in most cases, desired. However, increasing cracking temperature, pressure, and other conditions beyond conventional operating conditions generally lead to only a small increase in the EDC conversion at the expense of the selectivity to a vinyl chloride product. Furthermore, any outstanding increase in cracking temperature and pressure causes a drastic increase in coke formation.
Such coke formation in the pyrolysis cracker results in many problems. For example, coke formation inhibits the heat transfer to reactants in the pyrolysis cracker such that combustion energy is only partially transferred to reactants and the rest of the combustion energy is lost to the surroundings. Therefore, the pyrolysis cracker is required to be heated at a higher temperature to maintain the energy in the cracker at a sufficient level. Such heating requires more fuel and the lifetime of the alloy of the cracker is reduced. Conventionally, high temperatures cause erosion or corrosion of the walls of a cracker.
Meanwhile, the coke formed in the cracker reduces the width of the reaction path of EDC, thereby causing the pressure to drop with more depth when EDC passes through the cracker. As a result, more energy is required to compress the stream of a product, such as VC, in a downstream of the process. In addition, the coke reduces the effective inner volume of the cracker, which decreases the yield of the product and affects the selectivity of the reaction. Accordingly, more EDC is required to attain VC having a desired amount.
The coke formation also causes fouling of a heat exchanger and a transfer line exchanger (TLE.) A heat exchanger and a TLE remove as much thermal energy as possible from high-temperature products to stop any product degradation. However, when coke is formed in the heat exchanger and the TLE, heat transfer is inhibited. As a result, in the TLE, an increase of the pressure of gas existing in other transfer lines decreases, and in the heat exchanger, a decrease of pressure of a product stream more increases.
Accordingly, coke is periodically removed. Known methods for the removal of coke from pyrolysis crackers include controlled combustion or mechanical cleaning, or a combination of both methods. In the combustion process, a mixture of steam air of various steam/air ratios is admitted in the pyrolysis furnace at an elevated temperature, and the coke in the cracker is burnt out under a controlled condition. This process is conventionally referred to as hot decoke. For the mechanical cleaning, coke is physically chipped off the pyrolysis cracker inner surface and removed from the cracker. Both cracking and the hot decoke operations expose the pyrolysis cracker to a cycle between a HCl and chlorinated hydrocarbon-rich reducing environment and an oxygen-rich oxidizing environment at elevated temperatures, which causes corrosion and degradation of the pyrolysis cracker and shortens the cracker time.
The pyrolysis cracker is periodically decoked every 6 to 12 months, according to purity of reactant EDC and operating conditions, such as reaction temperature, reaction pressure, a feed speed of EDC, and a cracking depth. In particular, when a heat exchanger is installed in a high-temperature EDC pyrolysis cracker to efficiently use the energy at a cracker outlet, formation of a coke precursor results in a dramatic drop of a temperature in the cracker and thus coke is more quickly deposited on the inner walls of the heat exchanger, thereby shortening the removal cycle.
Conventional methods of inhibiting coke formation will now be described.
U.S. Pat. No. 6,228,253 teaches a method of coating Groups 1A and 2A metal salts on the inner walls of a cracker tube to inhibit coke formation. This method is advantageous in that there is no need to stop the process to remove the coke. However, this method can only be used for a conventional hydrocarbon pyrolysis reaction.
U.S. Pat. No. 3,896,182 teaches a method of inhibiting coke formation by lowering the oxygen content in the EDC feed.
U.S. Pat. No. 6,454,995 teaches a method of applying a phosphine-based compound (tributyl phosphine, triphenyl phosphine, or the like) to an EDC pyrolysis cracker. This method is not so effective on inhibiting coke formation and low reproducibility. In addition, since phoshpine-based compounds are expensive, the method is not cost effective.
Coke formation in pylolysis crackers continues to be undesirable and thus effective alternative methods to more efficiently inhibit the formation of coke during a pyrolysis process are always required.